Imagine any big city anywhere in the world without traffic just for a  day. Now, if that city were Paris, imagine further the photographic  possibilities, not to mention the visual, auditory and olfactory  potential.     Imagine no more because on September 27th, that’s just what Paris is going to do: “Une Journée Sans Voiture”  – A Day Without Car, for the first time in the city’s history.  City Hall calls it “a crazy gamble, but achievable.” No motorized  vehicle, with a few exceptions like ambulances, will be allowed to drive  the streets.    As Mayor Anne Hidalgo  announced in March : “Paris  will be completely transformed for a day. This is an opportunity for  Parisians and tourists to enjoy the city without noise, pollution and  therefore without stress.”   Other cities including Montreal, Bogota, Mexico City, Ho Chi Minh City  and Brussels have instituted Day Without Car programs, some of them  permanently and some partially, closin...